Wavelength Selective Switch (WSS) technology has recently enabled the introduction of multi-degree Reconfigurable Optical Add Drop Multiplexers (ROADM) and the deployment of cost-effective dynamic Wavelength Switched Optical Networks (WSONs). Several WSS-based ROADM structures have been proposed which differ in the way which add/drop functionalities are implemented, that is to say in the number of WSSs that they utilise. For example, a simple ROADM structure comprising no WSS guarantees low cost but imposes wavelength and directional constraints to each add/drop, having tributaries at fixed wavelengths and of fixed direction. More expensive ROADM structures, adopting one or two WSS per add/drop enables the add/drops to have tunable wavelength and/or configurable direction. The most commonly used ROADM structures comprise many add/drops of fixed wavelength and direction and a limited percentage of add/drops of partially or fully flexible wavelength and direction. The internet engineering taskforce, IETF, has proposed routing protocol extensions to describe internal ROADM structures in Y. Lee et al, “Routing and Wavelength Assignment Information Model for Wavelength Switched Optical Networks”, IETF draft, draft-ietf-ccamp-rwa-info-09, September 2010.
In the planning phase of configuring a communications network, where the hardware is defined to support a given traffic matrix, the real ROADM constraints are typically taken into account. The planning engine has complete knowledge of the configuration rules and limitations of the hardware and performs path computation taking all this information into account. In the case of dynamic lightpath setup, known methods of path computation commonly assume the use of ROADMs which have no wavelength or directional constraints, without accounting for node limitations. In a dynamic communications network, if node constraints are not considered during path computation and wavelength assignment, lightpath setup can easily be blocked and node resources are not properly exploited.